In recent years, there has been developed a so-called “active stabilizer system”, in detail, a stabilizer system in which stiffness or rigidity of a stabilizer bar is changed for thereby effectively controlling or restraining roll of a vehicle body. For instance, JP-A-2002-518245 disclose a stabilizer system equipped with an actuator which has an electric motor and which changes the stiffness of the stabilizer bar with respect to the roll of the vehicle body. By controlling the operation of the actuator, the roll of the vehicle body is effectively restrained or suppressed. JP-A-2005-35515 discloses, for controlling the stabilizer system, a method of determining an amount of electric power to be supplied to the electric motor based on roll-restraining moment to be exhibited by the stabilizer bar.